In my U.S. Pat. No. 3,532,679, issued Oct. 6, 1970, and entitled Hydrogels from Cross-linked Polymers of N-Vinyl Lactams and Alkyl Acrylates, I have described certain neutral hydrogels obtained by simultaneous polymerization and cross-linking of a mixture of an N-vinyl lactam, and alkyl acrylates. In my copending application Ser. No. 383,275 filed July 27, 1973, now U.S. Pat. No. 3,878,175 issued Apr. 15, 1975 I have described an improvement on my said patent, wherein a solution of the monomers in a hydrophobic solvent is simultaneously polymerized and cross-linked; whereby a highly absorbent, spongy, polymeric, neutral hydrogel is obtained.
I have now found that such N-vinyl lactam or other heterocyclic N-vinyl monomer based hydrogels can be modified, by incorporating cationic monomer in the mixture of monomers being simultaneously polymerized and cross-linked, so that a hydrogel having cationic functionality, and thus new and useful properties, is obtained.
A number of cationic polymeric materials, containing amino or quaternary ammonium groups which impart cationic functionality thereto, which are homopolymers or copolymers, with other ethylenically unsaturated monomers, are known in the art. One well known type of cationic polymers are the anion exchange resins available under such trade-names as Amberlite, Dowes, Permutit and Zeocarb. Other cationic polymers have found such varied applications in the arts as flocculants, as films and fibers (including use as additives to nonionic synthetic polymeric films and fibers) having improved dye receptivity, especially for acid dyes, as antistatic agents and a variety of other uses. Such known cationic polymers have however largely, if not always, been rather rigid solids and have not been hydrogels.
On the other hand practically all of the known hydrogels are neutral hydrogels and are not ionic in character. While in U.S. Pat. No. 3,689,634, issued Sept. 5, 1972to Kliment, Vacik, Majkus and Wichterle, entitled Protracted Activity Oral Hydrogel Bead; there is a broad suggestion that "it is also possible to replace the non-ionizable cross-linked hydrogels by physically similar hydrogels containing also ionizable groups"; the only examples of ionic hydrogels disclosed in this patent are: "A porous hydrogel capable of exchanging cations prepared by copolymerizing a mixture of 35 parts of methacrylic acid, and 30 parts of a 25 percent aqueous solution of maleic anhydride," disclosed in Example 8 at the top of column8 of the patent; and "A" copolymer prepared from 97 parts of ethylene glycol monomethacrylate, 2 parts of methacrylic acid and 1 percent of ethylene glycol bis-methacrylate by suspension polymerization in a concentrated, aqueous solution of sodium chloride, using 0.05 parts of diisopropyl percarbonate as a polymerization initiator," disclosed in Example9 at the middle of column 8 of the patent. These prior art ionic hydrogels are obviously substantially different from those of the present invention; inter alia, they are anionic in character and thus have the opposite charge of those of the present invention; in addition they contain no N-vinyl lactam, or other N-vinyl hetrocyclic monomer units and thus are structurally different from those of the present invention, and would be lacking in properties attributable to such N-vinyl lactam etc. units.
One of the outstanding advantages of the hydrogels of the present invention which contain cationic groups, as compared with the non-hydrogel form of cationic polymeric materials heretofore obtained by the use of cationic copolymerizable monomers, such as those mentioned above which contain amino or quaternary ammonium groups, is that the hydrogel form of the cationic polymers of the present invention permits and assures much more intimate contact between the cationic groups of the cationic polymeric hydrogel and any acidic material which it is desired to combine or complex therewith. In the presence of water the cationic hydrogels of the present invention are quite porous and swollen. Due to this swelling the water, and any acidic material dissolved or dispersed therein, of an aqueous medium with which these cationic hydrogels are used, or come in contact with during use, can readily diffuse or be transported throughout the hydrogel. As a result, combination or complexing of acidic materials with the cationic groups of the polymeric hydrogel can and does take place throughout the hydrogel in contrast for example, with the essentially surface action in the case of anion exchange resins. This swelling also increases the distance between the cationic groups of the hydrogel and this is also conducive to more complete reaction. Thus acidic materials can be combined or complexed much more efficiently and completely with the cationic groups of the cationic polymeric hydrogels of the present invention; and, conversely, acidic materials which are complexed or otherwise combined with the cationic groups of the cationic polymeric hydrogels of this invention may be more efficiently released therefrom and transferred to an aqueous medium with which they are used, especially in comparison with the corresponding cationic ion exchange resins.